I just built all this stuff, and I thought this stuff would be useful and turn up when people search it.
First some info on building rails in general. PVC pipes are very cheap(around $1-2 per 10' pipe depending on diameter. They are easily replaceable, so if one breaks(and if you use about 4 1" diameter pipes in a row next to each other, they wont break easily), all you have to do is unscrew the pipe, and put a new one on. It takes no more than 10 minutes.
Instructions on how to build the box are below, first for the rail:
It took me 1.5 hours to build this 10' rail.
Take 2 10' long 2x4's. For supports between them, cut 6 1' long pieces. Put them together in twos to form 3 supports. Screw the supports in between the 2 10' 2x4's in the ends and in the middle. Below each support, get a 3' or 4'(It really doens't matter) and put it horizontal under each support. Then, take a bunch(length isn't ver important) of 2x4's and cut them at 45 degree angles to fit them between the horizontal and verticle supports. Now all you have to do is take 2 10' long pvc pipes, put liquid nails on the top, and screw in the ends through the holes at the end.
This rail is really easy to make and it is pretty stable. You can change the dimensions to better suit what you like.
here's another picture:
For the box
You can use 2x4s or get someone to weld a frame for you. Since I dont know anyone that knows anything about welding, I used 2x4's.
To make a great box, you have to make a really frame that will support your topsheet, otherwise, you are stuck with a crap box and the topsheet may not hold up.
For the topsheet, you can use many materials:
HDPE(High Density Polyethylene)
bunch of pvc rails
Lexan is expensive. A 3/32 3'x4' sheet at home depot is $50. But 3/32 will likely not be thick enough and crack. You need 1/4. I had a very hard time finding 1/4 lexan. I looked in plastics stores everywhere, but they really only sold whole sheets at 8'x4' Your best bet when using lexan is buyyin it at www.polyzone.com or www.usplastics.com Shipping may be a bitch. Sometimes plastics stores may have scraps laying around that you can use. Look around at different plastics stores and you might be able to find some good stuff for free or cheap(same goes with HDPE)
HDPE(High Density Polyethylene) is what I have for a topsheet. It is usually a translucent frosty white, but I got a 8'x4' black sheet from a plstics store for $50 discounted because they couldn't sell it/use it. black is around 5% stronger than the regular white. It slides very well. The plastics guy said it would slide a lot better than lexan. It is either as strong as lexan or stronger. It is also around the same price range as lexan, but not found at home depot, but you can find it at polyzone or usplastics.
Puckboard is the lining around hockey rinks. sometimes rinks replace it and throw the old stuff out and you can pick it up for free. I know is slides well and some mountains use it for boxes. I don't know much about it other than that.
PVC sheets aren't very common, but you can buy them on plastics stores. They aren't as durable as HDPE or lexan, but they shouldn't be as expensive.
PVC pipes are the easiest to use. They are extremely cheap at home depot. For a 1 ft wide box, just get 12 1 inch pvc pipes and put them across. PVC pipes will slide well, althought they are soft and you can catch edges(not that big of a deal), but they aren't that durable. It took about 2 years for my PVC pipes to start cracking on my old rail set up. The great thing about PVC is that even though they aren't that strong, when they break, they are so cheap that you can go to home depot, get another pipe and just replace the old one. PVC pipes are the best solution for anyone looking to make a decent box that wont break the bank. But if you want a box at resort quality, look for puckboard HDPE or lexan
The basic things you need are verticle supports on the sides and ends, and horizontal supports holding up the topsheet and connecting the sides. You also need a layer of wood between the frame and the topsheet to help support the weight of the topsheet so it doesn't break. 1/2 thick chipboard will work. Oblique sopprts connecting verticle and horizontal supports may also be a good idea.
To connect the topsheet to the frame, it depends on what material you are using. For HDPE(and probn lexan), liquid nails and contact cement wont work at all(this is coming from the guy working at the plastics store), so you have to mechanically attach it by countersinking it onto the frame.
For coping, you dont really need it. It would help with protecting the box if you are doing 270 on and stuff, otherwise, the plastic naturally bevels out.
Basically just look at my box and copy the design. It's sturdy and works very well. The horizontal supports on the top and bottom, between the 12' long 2x4's are 9 inches. The vericle supports between the 12's are 1 foot. If you have any questions about anything else just post them. I really dont feel like posting exact directions on how to build it unless people will use it.
Hope this thread helps people out with this stuff. go and build a rail for the summer. you wont regret it.